Generally, a boiler is an apparatus designed to transfer combustion heat of fuel to water, etc., to thereby generate high-temperature and high-pressure steam. The boiler is widely used to supply the high-temperature and high-pressure steam to steam engines of thermal power plants and ships, etc., and other working and heating devices of various factories.
The boiler may be classified into various types based on a structure thereof. As one type of boiler, a once-through boiler, in the form of a forced circulation boiler, is made up of only pipes, including a water pipe and elongated bent pipe. The once-through boiler, so-called a forced once-through boiler, is operated such that water, supplied to one end of an elongated pipe, is heated, evaporated, and superheated in this sequence while passing through the pipe by pumping, thereby being discharged from the other end of the pipe as superheated steam. The once-through boiler is suitable to generate high-pressure steam, and especially, is advantageous for use as a super-critical pressure boiler.
In tandem with automatic control technique, the once-through boiler has been recently developed to achieve easy handling and high efficiency with small-capacity thereof. Also, the once-through boiler is able to generate high-pressure and high heat-capacity steam. For this reason, nowadays, the once-through boiler is widely used as a domestic or industrial boiler, for example, as a domestic heating boiler, spa equipment, or the like.
In the once-through boiler, water vapor, contained in combustion gas to be discharged to the outside, comes into contact with a heat exchanger to thereby be condensed. This enables recovery of latent heat of approximately 10%, realizing a high-efficiency apparatus.
However, the conventional once-through boiler has a problem in that it provides relatively low-temperature steam lower than 100 degrees Celsius. Since steam tends to gradually decrease in temperature as it is delivered to a heating device or engine, the relatively low-temperature steam of the conventional once-through boiler is insufficient to provide a desired standard temperature suitable for domestic or industrial facilities, resulting in a deterioration of heat efficiency thereof.
To obtain steam having a higher temperature, therefore, the conventional once-through boiler has been adapted to raise an internal pressure of a drum thereof. However, increasing the internal pressure of the drum or providing a high-strength water pipe deteriorates safety in operation of the boiler, causing accidents.